Show Agent tool subagents in tmux/iTerm2 split panes (like teammates)

Feature Request

Currently, teammateMode (tmux/iTerm2 split panes) only applies to agent teams (teammates). Subagents spawned via the Agent tool always run as invisible subprocesses — you only see the result when they finish.

Problem

When Claude spawns subagents (e.g., for research, implementation, or validation), there's no visibility into what they're doing. You just wait. This is especially frustrating for long-running subagents where you'd like to see progress, or when multiple subagents run in parallel.

Proposal

Extend teammateMode to also apply to subagents from the Agent tool. When teammateMode is "tmux" or "auto" (inside tmux), each subagent would get its own tmux/iTerm2 pane showing its live output — the same way teammates do today.

This could be:

• A separate setting (e.g., "subagentDisplayMode": "tmux")
• An extension of the existing teammateMode setting
• An opt-in flag on the Agent tool itself

Context

• Agent teams already support split panes via teammateMode: "tmux"
• The infrastructure for spawning panes exists — it just isn't wired to the Agent tool
• Users running Claude inside tmux (directly or via Neovim terminal) would benefit immediately